Diffusion of Sm-Nd in Scheelite and its Significance to Isotopic Dating and Tracing  

作　　者：ZHANG Dongliang Ian MCOULSON PENG Jiantang LI Shijie WANG Guoqiang 

机构地区：[1]Key Laboratory of Metallogenic Prediction of Nonferrous Metals,Ministry of Education,School of Geosciences and Infophysics,Central South University,Changsha 410083,China [2]Solid Earth Studies Laboratory,Department of Geology,University of Regina,Regina,Saskatchewan S4S OA2,Canada [3]Lunar and Planetary Science Research Center,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China [4]Xi'an Center of Geological Survey,CGS,Xi'an 710054,China

出　　处：《Acta Geologica Sinica(English Edition)》2023年第2期651-661,共11页地质学报（英文版）

基　　金：financially supported by the National Natural Science Foundation of China(Grant No.41403035);the National Basic Research Program of China(Grant No.2014CB440901)。

摘　　要：As the principal ore mineral in various tungsten(-gold)deposits,scheelite(CaWO_(4))plays an important role in directly dating the timing of ore formation,and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics.Since the retention of Sm-Nd systematics within scheelite is presently unconstrained,equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data.In order to better elucidate the closure of Sm-Nd in scheelite,the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein.The following Arrhenius relations were obtained:D_(Nd)=4.00exp(-438 kJ·mol^(–1)/RT)cm^(2)/s D_(Sm)=1.85exp(-427 kJ·mol^(–1)/RT)cm^(2)/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature.However,compared to other rare earth elements(REEs),which have markedly different atomic radii to either Nd or Sm,these are shown to exhibit a great variation in diffusivities.The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals,indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices.With this in mind,the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes.Based upon the diffusion data determined herein,Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics.Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits,and any later thermal environments.It is likely,therefore,that relevant isotopic information could be easily

关 键 词：Sm-Nd system DIFFUSION SCHEELITE closure temperature isotopic dating 

分 类 号：P618.67[天文地球—矿床学] P597.3[天文地球—地质学]